This proverb is all about the situations in which we feel lost, when everything goes wrong and we don’t know how to face the problems because it seems that everybody acts against us.
When a river recedes, fishermen can take advantage of this situation because fishes are more visible. In the same way, we should look at the good things in bad situations. Not always we are able to make disappear problems so quickly, but we can always learn something good from an unfortunate situation.
But the meaning is also that when we have troubles, other people win.